To improve the contrast of images of color cathode-ray tubes, fluorescent display tubes, etc., there have been known a method which employs a pigment-attached phosphor wherein filter particles capable of absorbing the reflection of an exterior light at a fluorescent screen, are attached to the phosphor surface (U.S. Pat. No. 3,886,394 and U.S. Pat. No. 4,307,320), and a method which employs colored glass as the face plate of a cathode-ray tube.
In the method which employs colored glass as the face plate of a cathode-ray tube, the transmittance of glass is reduced, whereby the contrast can be selected relatively readily within a practical range, but, the absorption of not only the exterior light but also the emission from the fluorescent layer increases, whereby the reduction of brightness is substantial.
Further, the method which employs the pigment-attached phosphor as the fluorescent layer is a method effective for improving the contrast without reducing the brightness of a screen as far as possible.
The above filter material is used sometimes for sharpening the emission spectrum of a phosphor. This filter material is required to have a transmittance high in the emission wavelength region of the phosphor and low in other wavelength regions. By this characteristic of the transmittance, light components which do not correspond to the emission of the phosphor itself will be absorbed, whereby the contrast under a bright exterior light can be improved.
With respect to the blue-emitting phosphor, it is suitable to use a pigment which has an absorption as small as possible at about 450 nm which is the emission peak of the phosphor, and an absorption as large as possible within the other wavelength regions.
As this sort of a blue pigment, there have been proposed ultramarine (3NaAl.SiO.sub.2.Na.sub.2 S.sub.2), prussian blue (Fe.sub.4 [Fe(CN).sub.6 ].sub.3.nH.sub.2 O), cobalt aluminate (CoO.nAl.sub.2 O.sub.3), cerulean blue (CoO.nSnO.sub.2), copper sulfide (CuS), etc. in Japanese Unexamined Patent Publication No. 28784/1979.
However, from researches of the present inventors, it has been found that the body color of cobalt aluminate usually practiced does not correspond to the emission spectrum of the blue-emitting phosphor. The reflectance of the cobalt aluminate is especially high at around 490 nm, such being ineffective to improve the contrast.
Further, the ultramarine practically used to some extent has excellent properties as compared with the cobalt aluminate, but its chemical stability is very poor, whereby a problem of fading in the attaching step to the phosphor or in the coating step to the cathode-ray tube is brought about.
To prevent the fading, it has been known to employ a method wherein the ultramarine pigment is covered with a silicic compound. However, when it is coated on a cathode-ray tube, there is a problem that its body color is remarkably changed by irradiation of electron beam, whereby a shift of the peak in the emission spectrum of the fluorescent screen is likely to occur.
Under these circumstances, it is intended in the present invention to solve the above problems and to provide a pigment-attached blue-emitting phosphor comprising a blue-emitting phosphor having a blue pigment attached thereto, which has fastness and chemical stability and is excellent in reflection spectrum characteristics, whereby good blue emission can be obtained when coated on the cathode-ray tube, and a color cathode-ray tube provided with a blue-emitting layer containing the above blue pigment on the inner surface of the face plate.